Native grasslands are exposed to anthropogenic nutrient inputs, especially from fertilization aimed to increasing biomass production for domestic animals. This practice has long-term consequences on the production through decreases in plant species richness and dominance of invasive, exotic species. However, little is known about the consequences on soil carbon sequestration, an important factor that may counterbalance carbon emissions and negative effects of climate change. In this work, we tested whether seasonal grazing by domestic livestock compensates the impact of fertilization on soil carbon stocks, and evaluated if nutrient additions and grazing differentially alter soil carbon stabilization. The study was conducted in a high-productive grassland located in the Flooding Pampa of Eastern Argentina. The design was a 2 x 2 full factorial experiment with grazing exclusion and fertilization as factors. Fertilizer (NPK) was applied to six 5 x 5 m plots, paired with unfertilized plots, both in grazed and exclosure areas (for a total of 24 plots). Total soil carbon (TSC), as well as soil organic matter, were estimated from 0-10 cm soil cores separated into fractions (particulate, POM, and associated to minerals, MAOM). We also measured soil respiration in laboratory incubations, and litter decomposition/stabilization rates using the tea bag method.
Grazing exclusion had a strongly negative effect on plant diversity (40%) and promoted dominance of exotic perennial grasses. Fertilizer addition similarly reduced plant richness in grazed and ungrazed plots by nearly 20% compared to pre-treatment levels. Fertilization marginally increased organic matter stabilization (p = 0.08) whereas nutrient addition significantly increased stabilization in the absence of grazing (p = 0.016). Indeed, both factors, grazing and nutrient additions, promoted litter decomposition although differences among treatments were not significant. Consequently, TSC and MAOM increased with nutrients additions when grazers were absent. Grazing stimulated carbon sequestration in the soil, whereas nutrients decreased the soil C:N ratio, thus increasing the labile POM fraction. Our results indicate that seasonal grazing mitigate the loss of diversity induced by fertilizer addition, presumably by reducing biomass production, and consequently balanced the stabilization effect induced by nutrients.